The present application relates to vehicle air braking systems, and is particularly directed to a bushing assembly for a spring brake actuator of a vehicle air braking system, such as for a spring brake actuator of a drum brake or a spring brake actuator of a disc brake of a vehicle air braking system.
A typical drum brake includes a brake shoe that engages a wheel drum to transmit braking torque through a brake spider to a wheel axle. One end of the brake shoe is pivotable against an anchor pin that is attached to one side of the brake spider. A cam follower roller is disposed at the other end of the brake shoe. When a service brake application occurs, a camshaft located on the side of the brake spider opposite the anchor pin is actuated to rotate about its longitudinal central axis. As the camshaft rotates about its longitudinal central axis, a cam surface of an S-cam mounted on the camshaft acts against the cam follower roller to pivot the brake shoe about the anchor pin. As the brake shoe pivots about the anchor pin, the brake lining of the brake shoe frictionally engages the wheel drum to transmit braking torque through the brake spider to the wheel axle and thereby to decelerate and brake the vehicle.
In a typical S-cam type of drum brake, a pneumatic system is linked by a pressurized air line to a spring brake actuator. The spring brake actuator is connected by a service push rod and clevis via a yoke to an arm of a slack adjuster. The slack adjuster is the link between the spring brake actuator and the camshaft. The slack adjuster includes a spline that is installed on a spline of the camshaft. The slack adjuster converts a linear force of the service push rod from the spring brake actuator into a torsional force that acts on the camshaft to rotate the camshaft about its longitudinal central axis. The entire slack adjuster operates as a unit, rotating as a lever with the camshaft as the brakes are applied or released.
More specifically, the service push rod from the spring brake actuator is disposed in a non-pressurized chamber of a service brake housing, and moves linearly (i.e., along the axial direction of the service push rod) in response to axial movement of an adapter push rod that is disposed in a pressurized chamber portion of a parking brake housing. When the pressure in the pressurized chamber portion of the parking brake housing drops, a compression spring in a non-pressurized chamber portion of the parking brake housing presses against a first diaphragm at one end of the adapter push rod and causes the other end of the adapter push rod to press against a second diaphragm. The second diaphragm in turn presses against one end of the service push rod to transfer the linear force from the adapter push rod through the second diaphragm to the opposite end of the service push rod that is fastened to the yoke of the slack adjuster.
A bushing assembly is usually disposed between the adapter push rod and an adapter base that is secured to the parking brake housing. The bushing assembly supports the adapter push rod for axial movement through an adapter plate that separates and lies between the parking brake housing and the service brake housing. The bushing assembly also provides a pressure seal between the pressurized chamber portion of the parking brake housing and the non-pressurized chamber of the service brake housing.
A typical disc brake converts air pressure into braking force. More specifically, when a foot brake is applied, compressed air enters through a supply port into a chamber of a spring brake actuator to apply pressure to a diaphragm. The pressure pushes the diaphragm, moving a pressure plate and push rod against a cup in a lever. The lever pivots on an eccentric bearing and transfers motion to an actuating beam. The actuating beam moves against a return force of a spring to move two threaded tappet sleeves and tappets. This movement of the two threaded tappet sleeves and tappets forces an inner brake pad into contact with a brake rotor. Further movement of the actuating beam forces a caliper, sliding on two stationary guide pins, away from the brake rotor. This movement of the caliper pulls an outer brake pad into the brake rotor to clamp the brake rotor in a braked position between the inner and outer brake pads to apply a braking force to a vehicle wheel which is attached to the brake rotor.
A bushing assembly is usually disposed between the push rod and a housing portion of the spring brake actuator. The bushing assembly supports the push rod for axial movement through an opening of the housing portion against the cup in the lever when the diaphragm is pushed in response to the foot brake being applied. Accordingly, those skilled in the art continue with research and development efforts in bushing assemblies for both spring brake actuators of disc brakes and spring brake actuators of drum brakes of a vehicle air braking system.